1. Field of the Invention
The present invention relates generally to a sensor signal integrating method and more specifically to a method of detecting angles by integrating angular rate signals generated from an angular rate sensor.
2. Description of the Prior Art
The background of the present invention will be explained with respect to its application to an angular rate sensor incorporated with a navigation system for an automotive vehicle.
Recently, a navigation system using an angular rate sensor has been put into practical use for an automotive vehicle. In this system, automotive vehicle travel locations are detected by an angular rate sensor and a distance sensor, and the detected vehicle locations are projected on a displayed map road so that the vehicle travel route is displayed along predetermined roads on the map. Since this system can be operative independently without need of other auxiliary means (e.g. radio signals), this system is effective in an urban area or other area subjected to radio interference where radio navigation systems cannot operate reliably.
FIG. 1 shows a block diagram of the navigation system using an angular rate sensor, in which the system comprises a distance sensor 1 for detecting vehicle travel distance and outputting a distance signal corresponding thereto; an angular rate (velocity) sensor 2 for detecting vehicle travel angular rate and outputting an angular rate signal corresponding thereto; a controller 3 (e.g. CPU) for calculating vehicle locations on the basis of the distance signals and angular rate signals; a memory unit 4 for storing various map data including branch points; a display unit 5 such as a CRT for displaying a road map and a vehicle travel location trace; and a keyboard 6 for entering data such as a start point on a road map.
The above controller 3 calculates vehicle travel angle on the basis of angular rate sensor signals as follows:
Assumption is made that an automotive vehicle turns to the right for a time period t.sub.1, goes straight ahead, and then turns to the left for a time period t.sub.2, as shown in FIG. 3(A). In this case, the angular rate sensor 2 generates two angular rate sensor signals f.sub.1 (T) and f.sub.2 (T) The controller 3 calculates a right-turn angle A.sub.R and a left-turn angle A.sub.L by obtaining areas S.sub.1 and S.sub.2 in accordance with the following expressions: ##EQU1##
FIG. 3(A) shows an example of an angular rate sensor signal in ideal form without being subjected to the harmful influence of drift in the output voltage level of the angular rate sensor 2. In practice, however, since drift is inevitably superimposed upon the output of the angular rate sensor 2 due to disturbance, the angular rate sensor signal shown in FIG. 3(A) changes to that shown in FIG. 3(B). In FIG. 3(B), the drift V.sub.0 (T) is approximate by a straight line .alpha.T for simplicity.
When an angular rate sensor signal on which drift is superimposed is integrated to obtain an angle, the area dotted in FIG. 3(B) causes an error.
In more detail, the area S.sub.1 shown in FIG. 3(A) changes to an area S.sub.1 ' enclosed by curves a, P.sub.1, b, c, d and A, so that area enclosed by a, b, c, d and A is added to the area S.sub.1 as an error. On the other hand, the area S.sub.2 shown in FIG. 3(A) changes to an area S.sub.2 ' enclosed by curves d, e and P.sub.2, so that an area enclosed by c, f, e and d is subtracted from the area S.sub.2 as an error.
Therefore, an actual vehicle travel route as shown by B in FIG. 2 is dislocated from the route B to an erroneous route A beyond an allowable range C shown by two dashed curves in FIG. 2 in a short period of time D. Once the calculated vehicle location deviates from the allowable range, the driver is required to manually reset or correct the current erroneous displayed location to a road displayed on the map to continue the normal display operation of the navigation system.
In other words, in the prior-art navigation system, when the output level of the angular rate sensor drifts or fluctuates, there exists a problem in that the driver often resets the erroneous displays by actuating a certain button, for instance.
Of course, it may be possible to solve the above-mentioned problem by using a high precision angular rate sensor to a certain extent. However, this results in another problem such that the sensor cost is extraordinarily high.